Field emitting device comprising field-concentrating nanoconductor assembly and method for making the same

ABSTRACT

This invention is predicated on applicants&#39; discovery that a highly oriented nanoconductor structure alone does not guarantee efficient field emission. To the contrary, the conventional densely populated, highly oriented structures actually yield relatively poor field emission characteristics. Applicants have determined that the individual nanoconductors in conventional assemblies are so closely spaced that they shield each other from effective field concentration at the ends, thus diminishing the driving force for efficient electron emission. 
     In accordance with the invention, an improved field emitting nanoconductors assembly (a “low density nanoconductor assembly”) comprises an array of nanoconductors which are highly aligned but spaced from each other no closer than 10% of the height of the nanoconductors. In this way, the field strength at the ends will be at least 50% of the maximal field concentration possible. Several ways of making the optimally low density assemblies are described along with several devices employing the assemblies.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/144,277 of identical title filed by the present inventors on Jul. 15,1999.

FIELD OF INVENTION

This invention pertains to field emitting devices and, in particular, tofield emitting devices comprising field-concentrating nanoconductorassemblies and to methods for making such devices.

BACKGROUND OF THE INVENTION

Field emitting devices are useful in a wide variety of applications. Atypical field emitting device comprises a field emitting assemblycomposed of a cathode and a plurality of field emitter tips. The devicealso typically includes a grid closely spaced to the emitter tips and ananode spaced further from the cathode. Voltage induces emission ofelectrons from the tips, through the grid, toward the anode.Applications include flat panel displays, klystrons and traveling wavetubes, ion guns, electron beam lithography, high energy accelerators,free electron lasers, and electron microscopes and microprobes. One ofthe most promising applications is thin, matrix-addressed flat paneldisplays. See, for example, Semiconductor International, December 1991,p.46; C. A. Spindt et al., IEEE Transactions on Electron Devices, vol.38, pp. 2355 (1991); I. Brodie and C. A. Spindt, Advances in Electronicsand Electron Physics, edited by P. W. Hawkes, vol. 83, pp. 1 (1992); andJ. A. Costellano, Handbook of Display Technology, Academic Press, NewYork, pp. 254 (1992), all of which are incorporated herein by reference.

A conventional field emission flat panel display comprises a flat vacuumcell having a matrix array of microscopic field emitters formed on acathode and a phosphor coated anode disposed on a transparent frontplate. An open grid (or gate) is disposed between cathode and anode. Thecathodes and gates are typically intersecting strips (usuallyperpendicular) whose intersections define pixels for the display. Agiven pixel is activated by applying voltage between the cathodeconductor strip and the gate conductor. A more positive voltage isapplied to the anode in order to impart a relatively high energy(400-5,000 eV) to the emitted electrons. For additional details see, forexample, the U.S. Pat. Nos. 4,940,916; 5,129,850; 5,138,237 and5,283,500, each of which is incorporated herein by reference.

A variety of characteristics are advantageous for field emittingassemblies. The emission current is advantageously voltage controllable,with driver voltages in a range obtainable from “off the shelf”integrated circuits. For typical CMOS circuitry and typical displaydevice dimensions (e.g. 1 μm gate-to-cathode spacing), a cathode thatemits at fields of 25 V/μm or less is generally desirable. The emittingcurrent density is advantageously in the range of 1-10 mA/cm² for flatpanel display applications and >100 mA/cm² for microwave power amplifierapplications. The emission characteristics are advantageouslyreproducible from one source to another and advantageously stable over along period of time (tens of thousands of hours). The emissionfluctuations (noise) are advantageously small enough to avoid limitingdevice performance. The cathode should be resistant to unwantedoccurrences in the vacuum environment, such as ion bombardment, chemicalreaction with residual gases, temperature extremes, and arcing. Finally,the cathode manufacturing is advantageously inexpensive, e.g. devoid ofhighly critical processes and adaptable to a wide variety ofapplications.

Previous cathode materials are typically metal (such as Mo) orsemiconductor (such as Si) with sharp tips. While useful emissioncharacteristics have been demonstrated for these materials, the controlvoltage required for emission is relatively high (around 100 V) becauseof their high work functions. The high control voltage increases damagedue to ion bombardment and surface diffusion on the emitter tips andnecessitates high power densities to produce the required emissioncurrent density. The fabrication of uniform sharp tips is difficult,tedious and expensive, especially over a large area. In addition, thesematerials are vulnerable to deterioration in a real device operatingenvironment involving ion bombardment, chemically active species andtemperature extremes.

Diamond emitters and related emission devices are disclosed, forexample, in U.S. Pat. Nos. 5,129,850, 5,138,237, 5,616,368, 5,623,180,5,637,950 and 5,648,699 and in Okano et al., Appl. Phys. Lett. vol. 64,p. 2742 (1994), Kumar et al., Solid State Technol. vol. 38, p. 71(1995), and Geis et al., J. Vac. Sci. Technol. vol. B14, p. 2060 (1996),all of which are incorporated herein by reference. While diamond fieldemitters have negative or low electron affinity, the technology has beenhindered by emission non-uniformity, vulnerability to surfacecontamination, and a tendency toward graphitization at high emissioncurrents (>30 mA/cm²).

Nanoscale conductors (“nanoconductors”) have recently emerged aspotentially useful electron field emitters. Nanoconductors are tinyconductive nanotubes (hollow) or nanowires (solid) with a size scale ofthe order of 1.0-100 nm in diameter and 0.5-10 μm in length. Carbonnanotubes, which are representative, are a stable form of carbon whichfeatures high aspect ratios (>1,000) and small tip radii of curvature(1-50 nm). These geometric characteristics, coupled with the highmechanical strength and chemical stability, make carbon nanotubesespecially attractive electron field emitters. Carbon nanotube emittersare disclosed, for example, by T. Keesmann in German patent No.4,405,768, and in Rinzler et al., Science, vol. 269, p.1550 (1995), DeHeer et al., Science, vol. 270, p. 1179 (1995), Saito et al., Jpn. J.Appl. Phys. Vol. 37, p. L346 (1998), Wang et al., Appl. Phys. Lett.,vol. 70, p. 3308, (1997), Saito et al., Jpn. J. Appl. Phys. Vol. 36, p.L1340 (1997), Wang et al., Appl. Phys. Lett. vol. 72, p 2912 (1998), andBonard et al., Appl. Phys. Lett., vol. 73, p. 918 (1998), all of whichare incorporated herein by reference. The synthesis of conductivenanowires based on semiconductor materials such as Si or Ge has alsobeen reported. See, for example, A. M. Morales et al. Science, Vol. 279,p. 208 (1998), which is incorporated herein by reference.

Nanoconductors are which are grown in the form of randomly oriented,needle-like or spaghetti-like powders that are not easily orconveniently incorporated into a field emitter device. Due to thisrandom configuration, the electron emission properties are not fullyutilized or optimized. Many nanoconductor tips may be buried in themass. Ways to grow nanoconductors in an oriented fashion on a substrateare disclosed in Ren et al., Science, Vol. 282, p. 1105 and Fan et al.,Science, Vol. 283, p. 512, both of which are incorporated herein byreference.

SUMMARY OF THE INVENTION

This invention is predicated on applicants' discovery that a highlyoriented nanoconductor structure alone does not guarantee efficientfield emission. To the contrary, the conventional densely populated,highly oriented structures actually yield relatively poor field emissioncharacteristics. Applicants have determined that the individualnanoconductors in conventional assemblies are so closely spaced thatthey shield each other from effective field concentration at the ends,thus diminishing the driving force for efficient electron emission.

In accordance with the invention, an improved field emittingnanoconductors assembly (a “low density nanoconductor assembly”)comprises an array of nanoconductors which are highly aligned but spacedfrom each other an average distance of at least 10% of the averageheight of the nanoconductors and preferably 50% of the average height.In this way, the field strength at the ends will be at least 50% of themaximal field concentration possible. Several ways of making theoptimally low density assemblies are described along with severaldevices employing the assemblies.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, advantages and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection with theaccompanying drawings. In the drawings:

FIGS. 1(a) and 1(b) illustrate pertinent features of conventionalnanoconductor assemblies;

FIGS. 1(c) and 1(d) show corresponding features of low densitynanoconductor assemblies;

FIGS. 2(a) and 2(b) are simulation plots of electrical potential nearthe field emission site at the nanoconductors of an assembly as afunction of the ratio of nearest neighbor distance (d) over the tubeheight (h).

FIGS. 3-9 schematically illustrate various techniques of makinglow-density nanoconductor assemblies; and

FIGS. 10-17 illustrate a variety of field emission devices using thelow-density assemblies.

It is to be understood that the drawings are for purposes ofillustrating the concepts of the invention and are not to scale.

DETAILED DESCRIPTION

This disclosure is divided into two parts. Part I describes the problemof poor nanoconductor field emission performance and discloses lowdensity nanoconductor assemblies as a solution to this problem, alongwith several methods of making the low-density assemblies. Part IIdescribes improved devices using the low-density assemblies and methodsfor making the improved devices.

I. Improved Nanotube Assemblies

Applicants have discovered that despite their promising features forfield emission, some conventionally fabricated nanoconductor assembliesperform poorly as field emission devices. For example, CVD grown carbonnanotube assemblies have highly oriented nanotubes of small diameter(0.8-1.3 nm) which provide effective field concentration and provide ahigh concentration of tube ends exposed to the assembly surface.Nonetheless emission results are poor. The devices exhibit high emissionthreshold fields and provide low current density.

Applicants have further discovered that nanoconductor assemblyperformance, including emission current density, can be enhanced byreducing the surface density of the nanoconductors. It was hypothesizedthat in conventional assemblies the density of highly orientednanoconductors was too high. The nanoconductors were too closely spacedand this close spacing reduced the field concentration at the emittingends.

Qualitatively, this problem can be visualized by reference to FIGS. 1(a)and 1(b) which schematically illustrate prior art nanoconductorassemblies having a high density of oriented conductors. The alignednanoconductors 10 extend from the surface of a substrate 11. In FIG.1(a) the nanoconductors have a high, relatively uniform surface density.In FIG. 1(b) the nanoconductors are in clusters 14 of high surfacedensity. The field concentration of each nanoconductor end is screenedand shielded by the presence of neighboring nanoconductors.

The shielding or screening effect of applied electric field fromneighboring nanoconductors is calculated and illustrated in FIGS. 2(a)and 2(b). These graphs plot the potential at the end vs. thespacing/height ratio (d/h) of the conductors. It can be seen that, ford/h>2.5, the potential at the end reaches at the highest possible level,indicating fullest field concentration at the end. For d/h<2.5, thepotential at the end starts to drop. At d/h=0. 1, the potential is onlyabout 65% of the maximum potential attainable for h=300 nm (FIG. 2a),and about 50% of the maximum potential attainable for h=100 nm (FIG.2b). This suggests that when the conductor spacing is less than 2.5times the height, neighboring conductors interfere with the fielddistribution and provide screening or shielding. As a result, the fieldconcentration is reduced, and the field strength falls toward theaverage field determined by the voltage and the average anode-cathodeseparation, thus eliminating the beneficial effect of thehigh-aspect-ratio nanoconductor geometry.

The simulation results of FIG. 2 suggest the use of low density alignednanoconductor assemblies, such as those shown in FIGS. 1(c) and 1(d).Here the low density spacing among individual conductors 10 (FIG. 1c) orsmall bundles 15 (FIG. 1d) meets the general condition of d/h>0.1. Inlow density assemblies the field strength at the emitting ends is atleast equal to or greater than ˜50% of the maximum field strengthachievable. The more desired d/h ratio is at least 0.2, preferably atleast 0.5, and even more preferably at least 1.0. The desired fieldconcentration at the emitting ends in the inventive structure isincreased at least by a factor of 5 and preferably at least by a factorof 20 as compared to dense prior art nanoconductor assemblies.

FIGS. 3(a)-3(d) schematically illustrate an exemplary method for makinglow density nanoconductor assemblies. As shown in FIG. 3(a),nanoconductor are typically deposited on substrate 11 in a mass 30 oftangled, high-aspect-ratio needles or fibers with random orientation.They can be prepared by a number of synthesis techniques including arcdischarge, chemical vapor deposition and laser ablation onto a substrateor by disposing (such as through spraying) pre-formed nanoconductorsonto a substrate. Single-wall carbon nanotubes typically exhibit adiameter of 0.8-6 nm and are often made in the form of a bundle.Multi-wall carbon nanotubes contain many concentric graphite sheets andtypically exhibit a diameter of 5-50 nm. The aspect ratio for both typesis typically 100-10,000, and both types have small-dimension,field-concentrating ends useful for electron emission.

The nanoconductors are preferably deposited on conductive substrates 11such as metals, conductive ceramics or polymers. Desirably, thenanoconductors exhibit good adhesion to the substrate, using, forexample, the techniques disclosed in copending U.S. application Ser. No.09/236,966 by Jin et al., filed Jan. 25, 1999 and entitled “ArticleComprising Enhanced Nanotube Emitter Structure and Process ForFabricating Article”, which is incorporated herein by reference.

The tangled mass 30 is then subjected to an external electric fieldapplied normal to the film surface during field emission. As shown inFIG. 3(b) loose nanoconductor ends 10 in this tangled structure willtend to stand up and align themselves with the electric field lines. Asa result, we have aligned nanoconductor ends 10 exposed in the field,the density of which is low enough to meet the d/h>0.1 criterion becauseof the random and tangled nature of the film 30. The ends can optionallybe further trimmed by, e.g. using a hot blade and spacer or using alaser beam to create aligned nanotube ends with equal height to furtheroptimize the emission properties. Such trimming is described in greaterdetail in copending U.S. application Ser. No. 09/236,933 by Jin et al.,filed Jan. 25, 1999 and entitled “Article Comprising Aligned, TruncatedCarbon Nanotubes and Process For Fabricating Article”, which isincorporated herein by reference. FIG. 3(c) illustrates trimming ofexcess portions 10A using spacers 31, and FIG. 3(d) shows the equalheight nanoconductors 10 produced by the trimming.

The average distance d and average height h for calculating the rationd/h may be defined analytically. In the case where the nanoconductorsare of substantially uniform length, the average distance${d = \sqrt{\frac{\sigma}{\pi}}},$

where σ is the areal number density of nanoconductors. This distance dis a typical distance from one nanoconductor to another. Where thenanoconductors are of substantially uniform length and are substantiallyperpendicular to the substrate, the average height h is simply theaverage length of the nanoconductors. If the lengths are uniform but notperpendicular to the substrate, h is the average distance of thenanoconductor tops from the substrate. Where the lengths aresubstantially nonuniform we can define an upper reference plane at adistance Z above the substrate, just below the tops of the tallest fewnanoconductors. (These are the nanoconductors that will be emittingalmost all the electrons.) Analytically we can calculate Z=({overscore(z_(i) ¹⁵)})^({fraction (1/15)}) where the heights Z_(i) are thedistances of the nanoconductor tops from the substrate and the mean{overscore (z_(i) ¹⁵)} is taken over all nanoconductors. The value for dis calculated, as above, from the areal density of nanoconductor endsabove the upper reference plane. We then define a lower reference planeat the mean height of the nanoconductor film. For a film where thenanoconductors are parallel and uniform, this plane is halfway up thenanoconductors. For a moderately disordered film, the plane is at aheight of Z=½Σ(Z_(i))²/ΣZ_(i) where the summations are taken over allnanoconductors. The average height of the nanoconductors h is then twicethe distance between the upper and lower reference planes. This resultsin d/h ratios for disordered films that correspond well to the valuesfor ideal ordered films. The parameters to be taken over allnanoconductors can, of course, be estimated by sampling in accordancewith accepted statistical practice.

FIG. 4 presents an alternative method of making low densitynanoconductor assemblies using a substrate 11 with pre-determinednucleation sites 40 exposed to the deposition environment. An exemplarysubstrate can be prepared by mixing metal powders, such as Cu, with finenanoparticles of Fe, Co, Ni or their oxide particles as catalyst fornanoconductor nucleation and growth. The size of these particles ispreferred in the range of 1-50 nm, more preferably in the range of 1-10nm range. The volume percentage of the catalytic particles is preferredless than 50%, and more preferably less than 30%.

The mixture is then pressed, sintered, polished and heat treated in areducing atmosphere such as H₂ or forming gas to reveal nanoparticles ofFe, Co or Ni on the surface as nucleation sites. Alternatively, thesurface reduction can be performed in-situ in the deposition chamberprior to the nanoconductor growth, to avoid re-oxidation of the metalcatalyst particles due to exposure to air. The nanoconductors will growon the exposed catalytic particles with a relatively low density, partlybecause of the controlled volume percentage of catalytic particlescontained in the alloy substrate and partly because of the substantiallylimited number of the catalytic particles exposed on a given sectionedor polished surface of the composite substrate material.

Another way of preparing a substrate with nucleation sites isillustrated in FIGS. 5(a)-5(d). As shown in FIG. 5(a), catalyticparticles 40 are suspended in a colloidal or dilute solution and sprayedthrough a nozzle 50 onto a substrate 11. The sprayed particles areoptionally diffusion bonded to the substrate by heat treatment toincrease the adhesion as illustrated in FIG. 5(b). A substantialfraction of the sprayed catalytic particles are then made invalid bycovering them with a non-catalytic layer 51 of metal (e.g. Cu), ceramic(e.g. SiO₂) or polymer to reduce the number of catalytic particles thatare exposed to the surface. This is shown in FIG. 5(c). In this way, thenucleation density of nanoconductors is reduced. The overlayer can bedeposited either by vacuum deposition (e.g. sputtering or evaporation)or by simple spraying. The overlayer can also be patternedlithographically, so that as shown in FIG. 5(d) the resulting lowdensity array of nanoconductors 10 is also patterned.

FIGS. 6(a)-6(d) present yet another way of preparing a substrate withnucleation sites. Here, the catalyst metals (such as Fe, Co, Ni) aredeposited onto a substrate as a thin film 60, e.g., a film with athickness in the range of 1-20 nm, by either chemical vapor deposition,electrochemical deposition or physical vapor deposition techniques. Thesubstrate 11 is preferably a conductive material with a large mismatchin lattice constant with respect to that for the catalyst metal film.The large mismatch will create large strains in the catalyst metal film.The deposited film is then subject to a high temperature heat treatment,e.g. 200-600° C. for 0.1-10 hours, preferably in an inert or vacuumenvironment. As shown in FIG. 6(b) formation of islands 61 will beinduced during this heat treatment in order to reduce the strain andoverall energy in the film 60. These fine islands 61 will serve asnucleation sites for the carbon nanotubes. To reduce the density ofnanoconductors, a fraction of these catalyst islands can be covered by anon-catalytic layer 51 as shown in FIG. 6(c). The nanoconductors 10 arethen selectively grown on these limited number of catalyst particles byusing known methods such as chemical vapor deposition (FIG. 6(d)).

The density of nanoconductors can also be controlled by selectivelyfilling catalyst particles 40 into a porous substrate 11A as illustratedin FIG. 7(a). Here, nanoporous materials such as porous silicon orsilica are used as substrate 11A with only a small fraction of itsavailable pores 70 filled with catalyst particles 40. The pores 70 arepreferably open pores, more preferably surface recessed pores, with thesize in the range of 1-100 nm. The filling of catalyst particles can beaccomplished by short-duration vacuum deposition, pressurized slurries,or via electrochemical means. As shown in FIG. 7(b) the nanoconductors10 will nucleate and grow only from the pores possessing catalystparticles. The nanoconductors will generally tend to align themselvesdue to the limitation placed by the vertically configured pore geometry.Electric fields can optionally be applied during the growth for furtheralignment.

Undesirably excessive filling of catalyst particles in the pores canoccur by either excessive pore filling or by doping of highly porousceramic or glass material. FIG. 8(a) illustrates such excessive porefilling. In such cases, partial coverage by non-catalytic films 51 canbe used to reduce the nucleation density of nanoconductors. FIG. 8(b)illustrates partial coverage by a non-catalytic film 51, and FIG. 8(c)shows the resulting low density growth of nanoconductors 10.

Another processing technique shown in FIGS. 9(a)-9(c) involves a poroussubstrate 11A (FIG. 9(a)). The nanopores 70 of the substrate are filledwith a diluted solution or slurry 90, e.g., aqueous or solvent solutioncontaining a catalyst metal (Co, Ni, Fe) in the forms of ions or slurrycontaining catalyst particles 40. The filling can be by spray depositionor by suction from the backside (see FIG. 9(b)). The solution or slurry90 is then heat-treated to decompose or burn-off the matrix solution andform catalyst islands 91 within the pores (FIG. 9(c)). The surface ofthe porous material is then polished mechanically or ion milled toremove any excessive amount of catalyst particles and decompositionproducts (FIG. 9(d)). Nanoconductors 10 are then grown from the reducednumber of nucleation sites dictated by the presence of catalystparticles (see FIG. 9(e)).

For certain field emission applications, the uniformity of emitterheight such as the height of aligned nanoconductors is important, partlyto avoid a catastrophic failure by shorting between cathode and anode orbetween cathode and gate, and partly to ensure maximumfield-concentration and efficient electron emission from the majority ofnanoconductor ends. The field-concentrating assembly structuresdescribed in FIGS. 5-9 can further be improved by trimming andequalizing the nanoconductor height, e.g., by using the method of FIG.3(c), to within 20% variation.

II. Improved Field Emitting Devices

The improved low-density nanoconductor assemblies are useful for varietyof devices, including microwave vacuum tube amplifier devices and flatpanel field emission display devices. Because efficient electronemission at low applied voltages is typically improved by the presenceof accelerating gate electrode in close proximity to the emitting source(typically about 1-10 μm distance), it is advantageous to have numerousgate apertures to enhance the capability of the emitter structure.Specifically, a fine-scale, micron-sized gate structure with numerousgate apertures is advantageous for attaining high emission efficiency.

FIG. 10 schematically illustrates a generalized field emission device100 comprising a low density nanoconductor assembly and a grid structure101 formed adjacent the assembly. The grid 101 is a conductive elementplaced between the electron emitting assembly and an anode 102. The gridis separated from the cathode 103 but is placed sufficiently close tothe nanoconductor emitter assembly to excite emissions (typically within10 μm of the emitting nanoconductor tips). This close spacing ispossible only if the emitter tips have relatively uniform height.

The grid 101 is generally separated from the cathode 103 by anelectrically insulating spacer layer 104 such as aluminum oxide orsilicon dioxide. Advantageously, the grid comprises an electricallyconducting layer, e.g., a thin film or thin foil, with a multitude ofapertures 105. Within each aperture, a multiplicity of nanoconductors 10emit electrons when a field is applied between the cathode and the grid.Insulating spacers 106 keep the anode and cathode spaced apart.

The dimension of the grid apertures 105 is typically in the range of0.05-100 μm in average maximum dimension (e.g., diameter),advantageously at least 0.1 μm, and more advantageously at least 0.2 μmfor ease of manufacturing. The average maximum dimension isadvantageously no more than 20 μm, more advantageously no more than 5 μmin order to increase the density of grid apertures and to reduce thevoltage necessary to achieve electron emission. Circular apertures areadvantageous in that they provide a desirable collimated electron beamwith relatively low perpendicular momentum spread. The thickness of thegrid conductor is typically in the range of 0.05-100 μm, advantageously0.05-10 μm. The grid conductor material is typically chosen from metalssuch as Cu, Cr, Ni, Nb, Mo, W or alloys thereof, but the use ofconductive ceramic materials such as oxides, nitrides, and carbides isalso possible. The apertured (or perforated) grid structure is typicallyprepared by conventional thin film deposition and photolithographicetching. Advantageously the grid is a high density apertured gatestructure such as described in U.S. Pat. Nos. 5,681,196 and 5,698,934,which are hereby incorporated herein by reference. The combination ofvery fine nanoconductor emitters with a high-density gate aperturestructure is particularly advantageous.

Such a high density gate aperture structure is conveniently formed usingthe particle mask techniques described in the aforementioned U.S. Pat.No. 5,681,196. Specifically, after formation of the nanoconductoremitter structure, mask particles (metal, ceramic, or plastic particlestypically having maximum dimensions less than 5 μm and advantageouslyless than 1 μm) are applied to the emitter surface, e.g., by spraying orsprinkling. A dielectric film layer such as SiO₂ or glass is depositedover the mask particles as by evaporation or sputtering. A conductivelayer such as Cu or Cr is then deposited on the dielectric whilemaintaining the mask particles in place. Because of the shadow effect,the emitter areas underneath each mask particle have no dielectric film.The mask particles are then easily brushed or blown away, leaving a gateelectrode having a high density of apertures.

FIG. 11 illustrates fabricating an emitter grid structure using the sucha particle mask technique. The mask particles 110 are located above theprotruding nanoconductor emitters 10. Upon deposition of the insulatinglayer 104 and the grid conductor layer 101, the mask particles 110 blockportions of the nanoconductor emitters 10. When the mask particles 110are removed, nanoconductors 10 are exposed through the resultantapertures. The resultant structure is then capable of being incorporatedinto a device.

FIG. 12 is a schematic cross section of a microwave vacuum tubeamplifier device—here a traveling wave tube (TWT) using the improvednanoconductor assemblies. The tube device contains an evacuated tube120, a source of electrons in the form of an electron gun 121, an inputwindow 122 for introducing a microwave input signal, an interactionstructure 123 where the electrons interact with the input signal, and amicrowave output window 124 where microwave power derived from theelectrons is taken out of the tube. In the case of a TWT, other desiredcomponents typically include a focusing magnet (not shown) to focus thebeam of electrons through the interaction structure 123, a collector 125to collect the electron beam after the output microwave power has beengenerated and an internal attenuator (not shown) to absorb microwavepower reflected back into the tube from mismatches in the output. For aTWT, the interaction region 123 is typically a conductive helix forbroadband applications and a coupled-cavity region for high powerapplications. The electron gun 121 is an electron source that generates,accelerates and focuses an electron beam to follow a desired trajectoryafter it leaves the gun.

FIG. 13 schematically illustrates a conventional electron gun comprisinga thermionic cathode 130, one or more grids 131 for inducing emission ofelectrons, focusing electrodes 132 for focusing the electrons into abeam, and-apertured anode 133 for further directing the beam 134 intointeraction structure 123. For TWT applications, a long, thin electronbeam at relatively low voltage and high current density is advantageous.Electron guns range in configuration from a planar cathode faced by aplanar anode to more elaborate designs such as Pierce guns, conicaldiode electrodes, concentric cylinders or spherical cap cathodes.

The cathode 130 and grid 131 are the source of electrons for theelectron beam in the TWT of FIG. 12. The cathode advantageously has thefollowing properties and capabilities: (1) exhibit a surface able toemit electrons freely without the necessity of external excitation suchas heating or bombardment, (2) supply a high current density, (3) longoperating life with its electron emission continuing substantiallyunimpaired, (4) allow production of a narrow beam with a small spread inelectron momentum, and (5) allow production of a modulated electron beamat or near the cathode. In contrast to conventional thermionic cathodes,cold cathodes comprising improved nanotube emitter assemblies exhibitthese properties. Specifically, nanoconductor-based cold cathodes arecapable of fast, room-temperature emission when an electric field isapplied. They allow the production of a modulated electron beam over adistance of a few microns (as in the case of beam modulation performeddirectly by the grids), permitting the use of a shortened interactionregion in the TWT tube design and resulting in a lighter, more compactdevice.

In operation of the device shown in FIGS. 12 and 13, an electron beam134 is accelerated from the cathode 130 by high voltages applied togrids 131 and anode 133. The electron beam is then shot into theinteraction structure 123 where it interacts with the microwave inputsignal such that the beam 134 is amplified as the electrons and thesignal travel together through the interaction structure 123. Theelectrons advantageously travel at the same velocity as the microwavesignal on the interaction structure 123. The power of the input signalmodulates the electron beam 134, and the modulated electron beam 134generates an amplified form of the input signal at the output 124.

When using nanoconductor-based cold cathodes in microwave vacuum tubedevices, it is desired to keep electron beam spread within a reasonablelevel. Electrons emerge from the cathode surface with a nonzero velocityand at various angles to the surface normal. The field-emitted electronsthus have a distribution of momentum values in the direction of electronbeam trajectory. These effects—random emission of electrons, undesirablemomentum perpendicular to the path from the cathode to the anode and theresulting crossing of electron trajectories on the microscopic scale—allreduce the performance of the microwave amplifier by giving rise to shotnoise as well as the minimum diameter that a convergent beam can attain.It is therefore desirable to inhibit electron beams from differentapertures in the grid from merging unless the electron beams are nearlyparallel. Specifically, if the beams merge while individually diverging,the phase space density of the resultant beam will be lowered, becauseat any given point electrons are found with a variety of differentmomenta.

It is possible to reduce the divergence angle of the electrons from eachaperture by creating an electrostatic lens in the aperture. However,Liouville's Theorem constrains the extent to which a lens is able toreduce the perpendicular momentum spread. If the emitting area is equalto the lens aperture, then no substantial improvement is obtained. Ifthe emitting area is smaller than the lens aperture, it is possible toreduce the perpendicular momentum distribution (with proper lens design)by the ratio of the radius of the emitting area to the radius of thelens.

It is therefore desirable to allow emission only from small spots nearthe center of each aperture, i.e. at most 70% of the area andadvantageously at most 50% of the area of the aperture. It is possibleto control the emission by patterning the substrate so that for aplurality of the emitting apertures, only a small area (smaller than theaperture area) is electrically conductive. It is also possible tocontrol emission by controlling the nanoconductor incorporation processsuch that only the central area within the emitting aperture isactivated and emits electrons, e.g., by depositing a non-emissiveoverlayer on the nanoconductor emitters everywhere but at the center ofthe apertures.

The invention provides an improved technique for reducing the divergenceangle. According to the invention, a multilayer, apertured grid is usedin which the first grid is operated at a negative potential. Themultilayer grid structure has at least two layers and advantageously atleast 4 layers of grid conductors, as illustrated in FIG. 14. Gridconductors 101A, 101B, 101C, 101D are separated by insulators 104A,104B, 104C, 104D, and define aligned apertures 140. Nanoconductoremitters 10 located within each aperture 140 are supported by a cathodeconductor 141, which is located on a substrate 11. The grid conductors101A-101D allow the electron beams to be focused during traveling. Thefirst grid layer closest to the emitters (101A) is generally biasednegative to reduce the perpendicular momentum through suppression offield emission near the edge of the grid apertures 140. A negative biason the first grid also focuses a diverging electron beam into one thathas momenta more nearly parallel to the surface normal. (A single gridprovides similarly useful properties if the field applied by the anodeis sufficiently large to force emission even in the presence of negativecharged grid. However, multiple grids are advantageous in reducing therequired voltage on the anode, and in providing a better collimatedelectron beam.)

The first grid is typically 0.05 to 10 of its average maximum aperturedimension (e.g., diameter in the case of round apertures) above thecathode, advantageously 0.3 to 2. Typically, the apertures are round andhave a diameter of 0.05 to 100 μm, advantageously at least 0.1 μm, moreadvantageously at least 0.2 μm. This first grid reduces the electricfield at the cathode surface, near the edge of the hole, and therebysuppresses emission preferentially from the edge. Successive gridstypically exhibit positive voltages relative to the cathode.

The multilayered grid structure can be prepared by conventional thinfilm deposition and photolithographic techniques. It is also possible toprepare the grid structures of FIG. 14 by a particle mask technique asdiscussed previously and illustrated in FIGS. 15 and 16. The thicknessof the grid conductor layers 101A-101D is typically in the range of 0.05to 100 μm, advantageously 0.1 to 10 μm. The grid conductor layers aregenerally selected from a metal such as Cu, Cr, Ni, Nb, Mo, W, or alloysthereof, but the use of conductive ceramics such as oxides, nitrides,and carbides is also possible. The insulator layers 104A-104D aretypically formed from materials such as silica or glass.

In FIG. 15, the mask particles 150 are typically ferromagnetic (e.g. Fe,Ni, Co, or their alloys). Desirable particle size is typically in therange of 0.1-20 μm in average diameter. During the placement of theparticles, e.g. by sprinkling onto the nanotube emitter structure, avertical magnetic field is applied, which causes the ferromagneticparticles 150 to form a vertically elongated chain-of-spheres containingat least 2 particles. Some chains-of-spheres may have more particlesthan others, but this does not affect the process of depositing themultilayer grid structure. After alternating deposition of insulatingspacer film (104A-104D) and the grid conductor film (101A-101D) intomultilayer stacks, the magnetic field is removed. The ferromagneticparticles 150 are then also removed, e.g., by magnetically pulling awayfrom above using a permanent magnet or electromagnet, or by chemicaletching.

An alternative particle mask approach is schematically illustrated inFIG. 16. In this approach, elongated or prolate ferromagnetic particles160 are sprinkled in the presence of vertical magnetic field so thatthey stand up vertically due to shape anisotropy and to serve as maskparticles during the subsequent deposition of the multilayer gridstructure (100A-100D and 101A-101D), conductor layer 11 andnanoconductor emitters 10. For convenience, the conductor/nanoconductorassembly can be supported on a larger substrate 141.

The elongated mask particles 160 typically have an average axial maximumdimension, e.g., diameter, in the range of 0.1-20 μm, and a length todiameter aspect ratio of at least 2. It is possible to prepare theparticles 160, for example, by thin film deposition (e.g. by sputtering,evaporation, electroless plating) of the mask material through aperforated template (not shown) placed at a desired height above thenanoconductor emitter layer. Suitable materials for this type ofelongated mask particles 160 include metals such as Cu, Al, Ni, easilywater or solvent dissolvable polymers (e.g., polyvinyl acetate,polyvinyl alcohol, polyacrylamide, acrylonitrile-butadiene-styrene orABS), volatile polymers (e.g., PMMA), or easily dissolvable salts (e.g.,NaCl). After deposition of the particles, the template is removed, andthe multilayer grid structure is formed by deposition over the maskparticles. The mask particles are then dissolved away to expose theaperture.

The cathode and gate structure of FIG. 14, as used in a microwaveamplifier, is not necessarily flat in surface geometry. It is possibleto use a reshaped bulk nanoconductor composite emitter, or a curvedsubstrate having thin film array emitters deposited thereon. The curvedsubstrate is prepared, for example, by etching or mechanical polishing(e.g., in the case of materials such as Si) or by plastic deformation(e.g., in the case of ductile metals such ad Cu, Mo, Nb, W, Fe, Ni, oralloys thereof).

Advantageously, the nanoconductor-containing cathode and multilayer gridstructure of FIG. 14 is used in a TWT, instead of a thermionic emissioncathode. Also, the cathode/grid structure of FIG. 14 is advantageouslyslightly concave for the purpose of focusing the emitted electrons intoa beam.

The nanoconductor emitter structures of FIGS. 13 and 14, reduce theperpendicular momentum spread of electrons emitting from the cathode dueto four features. (1) Low voltage emission is conducive to reduced beamspreading. If the emitter geometry is held constant, the perpendicularmomentum spread scales as the square root of the emission voltage. Theuse of field-concentrating, low-density protruding nanotube emittersprepared according to the invention allows low voltage emission andhence reduced perpendicular momentum in microwave amplifier operation.(2) Electron emission is restricted to the central area portion, whichis much smaller than the entire grid aperture area. (3) The electronbeam is focused by the stack of the multilayer grid structure. (4) Aconcave substrate further focuses the electron beam.

It is also possible to use the nanoconductor-based emitters of theinvention to fabricate a flat panel, field emission display. Such afield emission display is constructed, for example, with a diode design(i.e., cathode-anode configuration) or a triode design (i.e.,cathode-grid-anode configuration). Advantageously, a grid electrode isused, more advantageously a high density aperture gate structure placedin proximity to the nanoconductor emitter cathode, as discussedpreviously.

For display applications, emitter material (the cold cathode) in eachpixel of the display desirably consists of multiple emitters for thepurpose, among others, of averaging out the emission characteristics andensuring uniformity in display quality. Because of the nanoscopic natureof the nanoconductors, the emitter provides many emitting points,typically more than 10⁴ emitting tips per pixel of 100×100 μm², assuming0.01-1% areal density of nanoconductors with a tubular diameter of 5-100μm. Advantageously, the emitter density in the invention is at least1/μm², more advantageously at least 10/μm². Because efficient electronemission at low applied voltage is typically achieved by the presence ofaccelerating gate electrode in close proximity (typically about 1 microndistance), it is useful to have multiple gate apertures over a givenemitter area to utilize the capability of multiple emitters. It is alsodesirable to have fine-scale, micron-sized structure with as many gateapertures as possible for increased emission efficiency.

FIG. 17 illustrates a flat panel field emission display using ananoconductors emitter structure of the invention. The display containsa cathode 11 including a plurality of nanoconductor emitters 10 and ananode 102 disposed in spaced relations from the emitters 10 within avacuum seal. The anode conductor 102 formed on a transparent insulatingsubstrate 170 is provided with a phosphor layer 171 and mounted onsupport pillars (not shown). Between the cathode and the anode andclosely spaced from the emitters is a perforated conductive gate layer101. Conveniently, the gate 101 is spaced from the cathode 11 by aninsulating layer 104.

The space between the anode and the emitter is sealed and evacuated, andvoltage is applied by power supply 172. The field-emitted electrons fromthe nanoconductor emitters 10 are accelerated by the gate electrode 101,and move toward the anode conductor layer 102 (typically a transparentconductor such as indium-tin oxide). As the accelerated electrons hitthe phosphor layer 171, a display image is generated.

It is to be understood that the above-described embodiments areillustrative of only a few of the many possible specific embodimentswhich can represent applications of the principles of the invention.Numerous and varied other arrangements can be readily devised by thoseskilled in the art without departing from the spirit and scope of theinvention.

What is claimed is:
 1. In an assembly of nanoscale conductors comprisinga substrate and a plurality of nanoscale conductors attached to thesubstrate, the conductors extending from a substrate surface to tips andhaving diameters in the range 1.0-100 nm and lengths in the range0.5-100 μm; the improvement wherein the average separation distancebetween adjacent nanoscale conductors is in the range 0.1 to 2.5 timesthe average conductor height above the surface.
 2. The assembly of claim1 wherein the nanoscale conductors are carbon nanotubes.
 3. The assemblyof claim 1 wherein the nanoscale conductors are semiconductor nanowires.4. The assembly of claim 1 wherein the substrate surface is planar andthe conductors are substantially perpendicular to the planar surfacewith an average deviation of alignment less than 30° from theperpendicular.
 5. The assembly of claim 1 wherein the conductors aresubstantially equal in height to within 20% of the average height.
 6. Inan electron field emitting device comprising a cathode, a plurality offield emitter tips, a grid spaced relatively close to the emitter tipsand an anode spaced relatively farther from the tips, the improvementwherein the field emitting device comprises the assembly of claim 1 withthe substrate of the assembly comprising the cathode and the tips of theconductors comprising the emitter tips.
 7. In a microwave vacuum tubeamplifier comprising an evacuated tube, a source of electrons within thetube, an input for a microwave signal, an interaction structure withinthe tube for interacting the input signal with electrons and an outputfor an amplified microwave signal; the improvement wherein the electronsource comprises a field emitting device according to claim
 6. 8. In adisplay device comprising a cathode including a plurality of electronemitters, an anode disposed in spaced relation to the emitters, theanode including a phosphor layer, and a gate disposed between theemitters and the anode, the improvement wherein the display devicecomprises the assembly of claim 1 with the substrate of the assemblycomprising the cathode and the tips of the conductors comprising theemitters.